CCL21 and primary cutaneous T-cell non-Hodgkin lymphoma: Key chemokine-receptor signaling axes within the CC chemokine family include CCL17/CCL22-CCR4, CCL18-CCR8, CCL19/CCL21-CCR7, CCL2-CCR2, CCL27/CCL28-CCR10, CCL20-CCR6, and CCL5-CCR5, which play significant roles in the immune regulation of CTCL.